1. The Field of the Invention
This invention relates to a high speed data modem for transmitting and receiving data over common, band-limited telephone channels and the like, and more particularly to a modulation method and apparatus for multicarrier data transmission which avoids smearing and distortion of the data being transmitted and interframe interference thereby allowing high speed and error free data transmission.
2. The Prior Art
Modems (an acronym for modulation/demodulation) were developed to enable the transmission of digital data over low-grade, but pervasive, analog telephone audio channels. Such channels generally have a bandwidth of less than 3,000 Hz compared to the audio frequency spectrum covering from 20 Hz to 20,000 Hz. With the ability to transmit data over telephone channels, not only was the need for specially designed data transmission networks eliminated, or at least reduced, but also the number of possible originating and destination stations was greatly multiplied, limited only by the availability (or more precisely non-availability) of a telephone line.
Modems operate by transforming binary (two-level) digital data signals into analog signals suitable for transmission over telephone channels and then, after transmission, transforming the analog signals back into the digital data signals. This conversion involves modulating or encoding the digital data onto a carrier signal, or carrier signals, at the transmitting end, and demodulating or decoding the transmitted signals at the receiving end to recover the desired digital data.
Since the introduction of modems, development efforts have been directed to improving their speed and accuracy, resulting in systems capable of transmitting 9600 bits per second (see, for example, U.S. Pat. Nos. 4,206,320 and 4,771,417) and, most recently, 14,400 bits per second (see U.S. Pat. No. 4,646,305). Also see U.S. Pat. Nos. 3,955,141, 4,085,449, 4,355,397, 4,514,825, 4,653,044, 4,686,690, 4,734,920 and 4,796,279.
Among the currently used methods of data transmission over telephone lines are those which simultaneously utilize several carriers. Such multiple carrier data transmission techniques are attractive for a number of reasons. For example, multiple carrier techniques allow the most efficient use of the available bandwidth, especially when signal/noise ratios vary across the passband of the communication channel. Also, as the performance of digital signal processors increase and their prices decrease, multi-carrier modulation systems become more attractive alternatives to other systems.
One of the problems which inhibits improvement in the speed and accuracy of data transmission over telephone channels, at least for those systems which utilize multiple carriers (frequencies), is what is denoted "group delay distortion". This type of distortion comes about because of the difference in phase delay for different frequencies on communication channels. The result of such distortion is that different frequencies of a transmitted composite analog signal arrive at the receiving end at different times, some frequencies lagging behind others, so that one signal symbol or frame may interfere with an immediately preceding or succeeding symbol, e.g., the late arriving frequencies of a symbol interfering with the early arising frequencies of a succeeding symbol.
Another problem of systems which utilize multiple carriers arises from the fact that the symbol waveforms (transmitted composite analog signal) are periodic and thus, if transmitted continuously, one immediately after the other, give rise most times to sharp discontinuities between symbols. These discontinuities, in turn, produce severe distortion (from the harmonics of the discontinuity) in the succeeding symbol.
Even in the case of multicarrier QAM (quadrature amplitude modulation) the problem of smearing between frames and interframe interference caused by interframe discontinuities and group delay can be severe. In QAM. successive bits to be transmitted are converted (through a straight binary code, gray code, or other means) into a number of multilevel signals. Each transmitted signal is made up of an inphase (cosine) component and a quadrature (sine) component; thus, the total number of multilevel signals transmitted is equal to twice the number of carrier frequencies.
When using QAM, in order to maintain orthogonality of the carriers, the amplitude of each must remain constant for a period equal to the reciprocal of the frequency difference between adjacent carriers; this period is referred to as a "frame". Importantly, when demodulating the QAM signal, the received signal must be integrated for precisely one frame period in order to separate the individual carrier components.
While the integration must be carried out precisely on each frame, when the signal is transmitted through a communications channel, the beginning and the end of the frame are "smeared" in time due to the inadequate channel transient response. Smearing has two effects: First, time domain components of the frame located near it's boundaries are not properly included in the frame integration, causing loss of orthogonality between carriers or intercarrier interference, as well as a loss in amplitude response; and, Second, time domain components of adjacent frames near the boundaries are improperly be included in the frame integration, causing interframe interference.
One approach to overcoming these problems is described in U.S. Pat. No. 4,206,320 for example, is to provide a gap or guard time between symbols to thus reduce intersymbol and discontinuity distortion. That is, the demodulator at the receiving end is arranged to ignore the received signal for a portion of the baud time. Of course, this also increases the transmission time (i.e., increases delay) which, stated in other words, decreases the transmission rate.
Another approach to solving the problems experienced in the prior art is described in U.S. Pat. No. 4,943,980. U.S. Pat. No. 4,943,980 the rotation of, or circularly shifting of, each frame waveform in such a manner as to minimize the discontinuities between frames, and thus minimizing the effects of the channel transient response. This approach may not be effectively carried out for all possible frame waveforms, and even if it can be, there remains the requirement for conveying the amount of rotation of each frame to the receiver. Since information conveying the amount of the rotation of each waveform must be sent at the expense of the data to be transmitted, there is again a loss in channel capacity. U.S. Pat. No. 4,943,980 discloses an apparatus and method for a high speed modem which overcomes the above described problems but utilizes a frame rotation scheme where the amount that each frame is rotated must be conveyed from the transmitter to the receiver.